Heterogeneous networks (HetNets) are an appealing solution for providing increased capacity to mobile users. FIG. 1 illustrates such heterogeneous network 1, comprising a macro base station 2 providing a wide area coverage serving users within a macro cell 3, particularly for users moving at high speed. Within the macro cell 3 several low power nodes are employed in service areas having a higher density of users requiring high data rates, or hotspots as such areas are denoted. Examples of such low power nodes comprise pico base stations 5 within pico cells 6, relay base stations 8 providing relay cells 9 and femto/home base stations 4 covering femto cells 7. In the heterogeneous network thus, the macro base station 2 could be used for coverage and femto or pico base stations 4, 5 for capacity.
In communication systems, e.g. in the heterogeneous network, a physical cell identity (PCI) is used in a radio layer for identifying different cells within the communication system and in neighboring cell measurements and measurement reports. Global cell identities are mainly used in other layers, for example in handover routing. The total number of different PCIs is limited in various communication technologies. In Long Term Evolution (LTE) for example, primary and secondary synchronization signals together encode the different LTE cells' PCI, which is composed of the physical layer cell identity group (0, 1, . . . , 167) and the physical layer identity (0, 1 or 2). The total number of PCIs, in LTE, is thus 504.
Typically the PCI allocation is a demanding task in network planning in order to ensure that neighboring cells do not use same PCI. The PCI should be unique and enough for identifying the cell.
In HetNet deployment, the above may create specific issues, as the HetNet cells 5, 7, 9 also are identified by a PCI on the radio layer. In order to provide a unique PCI, some of the available different PCIs could be dedicated for HetNet cells 5, 7, 9. However, as the number of PCIs is limited it would be difficult to dedicate a suitable number of them to HetNet cells 5, 7, 9. This leads to a situation commonly known as “PCI confusion”, which means that the use of only PCI is not enough to uniquely identify a cell, and a global cell identity (e.g. E-UTRAN Cell Global Identity, E-CGI) is required to uniquely identify the HetNet cell.
FIG. 2 illustrates an exemplary case of the above described PCI confusion. In this exemplary case there exists two different pico cells 11, 12 within the coverage area of the macro cell 14, and these pico cells have been configured with the same PCI, which is 101 in the illustrated case. When the user equipment (UE) 10 approaches a HetNet cell 11 it will indicate this to its serving macro base station 13 controlling a macro cell 14. This indication is conveyed in a measurement report and one included information element in this measurement report is the PCI (101). A “PCI confusion” situation may occur in the macro base station 13 and in the following some different possible scenarios are mentioned:
a) The macro base station 13 is not aware of the “PCI confusion” possibility and has stored neighbor relation for the PCI 101 towards the HetNet cell 12. In this case, any triggered handover towards the HetNet cell 12 will fail as the handover preparation was performed towards HetNet cell 12 and the UE 10 attempted to perform handover to HetNet cell 11.b) The macro base station 13 is not aware of the “PCI confusion” possibility and has stored neighbor relation for the PCI 101 towards the HetNet cell 11. In this case, any triggered handover towards the HetNet cell 11 would anyhow succeed.c) The macro base station 13 is aware of the “PCI confusion” possibility. In this case the macro base station 13 needs to ask the UE 10 to report more information that can be used for uniquely identifying the HetNet cell 11. One possibility is to use Automatic Neighbor Relation (ANR)-like principles to solve the PCI confusion.d) The macro base station 13 is aware of the “PCI confusion” possibility and knows all cells 11, 12 with that particular PCI (101) and has stored neighbor relation lists for all of them. The macro base station 13 requests a handover preparation to all possible target cells 11, 12 and sends a handover command to the UE 10 for one of the cells, for example cell 12. When the UE 10 approaches cell 11 the plain handover will fail, but an RRC Re-establishment procedure will follow, so the service is resumed after only a short interruption. However, this method is only practical for cases in which there are only a few cells having the same PCI and it would often cause an interruption.
One known solution to this PCI confusion problem is thus to let the UE provide more information to the serving cell enabling it to uniquely identify the target cell, for example by applying principles similar to ANR. One such procedure is called “inbound mobility to E-UTRAN CSG cells” in 3GPP Technical Specification 36.300. However, “inbound mobility” has some drawbacks, particularly if used in every case when a PCI confusion might arise. The frequent use of ‘inbound mobility’ may create interruption in the ongoing transmission in the serving cell as the UE needs long gaps to read the E-CGI from the target cell. The use of ‘inbound mobility’ may also create delay in the handover process as the UE needs to retrieve additional information from the target cell. Further, the support of ‘inbound mobility’ may be implemented as an optional feature, resulting in that it would not be available for use with all UEs.
From the above, it is clear that there is a need for improvement in this regards.